1. Field of the Invention
This invention relates to a rear-suspension system for a vehicle, and more particularly to an improved rear-suspension system for a vehicle, exhibition an excellent toe-in effect.
2. Description of the Prior Art
As is well known, the rear-suspension system for a vehicle is desired to be arranged to that the rear wheels or the tires thereon are caused to toe in during travel, especially during cornering, in order to improve driving stability, driving comfort and the like. The centrifugal force exerted on the vehicle body during cornering acts on the rear-suspension as a lateral force. The tires are desired to counteract the lateral force with an increased resisting force in order to maximize the critical acceleration G in turning. The resisting force against the lateral force can be increased by causing the tires on the rear wheels to toe in to create a slip angle. By increasing the resisting force, the roadgripping force of the rear tires can be improved and the tendency to understeer can be enhanced, whereby the driving stability of the vehicle is improved. When the accelerator pedal is pushed down during cornering, a driving force is exerted on the tires, while when the accelerator pedal is released during cornering, a braking force is exerted on the tires. The tires tend to toe out upon release of the accelerator pedal which has been depressed, while they tend to toe in upon depression of the accelerator pedal. This causes the tires to toe out and in during cornering, thereby adversely affecting the driving stability of the vehicle. Further, as the rubber bushings for improving driving comfort are disposed inside the treading point of the tires, the braking forces exerted on the tires when the brake pedal is depressed or an engine-brake effect occurs cause the tires to toe out and accordingly the driving stability is lowered. This means that the driving stability is lowered as the driving comfort becomes higher since the softer the rubber bushings are, the more comfortable the vehicle is to ride in. Therefore, there is a demand for a rear-suspension system which can cause the rear tires to toe in even when braking forces are exerted thereon through operation of the brake pedal or by the engine-brake effect. The ability of the rear-suspension system to always cause the tires to toe in (This ability will be referred to as "Toe-in ability" hereinbelow.) ensures a good driving stability during cornering. The toe-in ability of the rear-suspension is also desirable from the viewpoint of the stability of the vehicle during straight travel at a high speed which is particularly required in the case of a sports car. Actually, roads are not completely flat but inherently have bumps and recesses of various sizes which act on the tires as external disturbances in various directions. Further, winds impinging upon the vehicle body in various directions also act on the tires as external disturbances in various directions, and in particular, side winds act on the tires as lateral forces. If the rear-suspension can maintain its toe-in ability even when these external disturbances are exerted on the tires, an under-steer condition of the vehicle is always ensured, whereby the vehicle can always be stabilized. The external disturbances act as the lateral force, braking force or driving force described above irrespective of their origins.
Therefore, the rear-suspension system is desired to be able to keep its toe-in ability against any one of the lateral force, braking force (during either the operation of the brake pedal or the engine-brake effect), and driving force. The lateral force is typically a thrust load produced during cornering and comprises a force acting on the treading point of the tires from outside to inside. The braking force due to operation of the brake pedal comprises a force acting on the treading point of the tires from front to rear while the braking force due to the engine-brake effect comprises a force acting on the wheel center of the tires from front to rear. The driving force comprises a force acting on the wheel center from rear to front. The four forces, the acting point thereof and the acting direction are tabulated in the following table.
______________________________________ force acting point direction ______________________________________ lateral force treading outside to point inside brake force treading front to point rear engine brake wheel center front to force rear driving wheel center rear to force front ______________________________________
In the above table and the following description, the braking force due to operation of the brake pedal and the same due to the engine-brake effect are referred to as "brake force" and "engine-brake force", respectively, in order to clearly distinguish them from each other.
There have been developed various rear-suspension systems which can keep the toe-in ability against the lateral force produced during cornering. For example, there is disclosed in Japanese Patent Publication No. 52(1977)-37649 such a rear-suspension system which utilizes three rubber bushings of different hardnesses: In West German Patent Laid Open Nos. 2,158,931 and 2,355,954 there are disclosed such rear-suspension systems in which each wheel hub is supported by way of a vertical shaft and a spring. However they are fairly complicated in structure. Further, the rear-suspension systems of the type described above cannot keep their toe-in ability against all of the four forces described above, but only against the lateral force.